Recently, capacitive electromechanical transducer apparatuses manufactured by performing a micromachining process have been studied actively. General capacitive electromechanical transducer apparatuses include cells that include a lower electrode, a vibrating membrane that is supported and arranged with a predetermined space from the lower electrode, and upper electrodes arranged on a surface of the vibrating membrane. These capacitive electromechanical transducer apparatuses are used as, for example, capacitive micromachined ultrasonic transducers (CMUTs).
A CMUT performs at least either conversion of an electric signal into an ultrasonic wave or conversion of an ultrasonic wave into an electric signal by using a lightweight vibrating membrane. A CMUT can be easily designed so as to have a wide frequency band property in both liquids and the air. A CMUT makes it possible to perform medical diagnoses that attain higher accuracy than previous medical diagnoses, and thus a CMUT is receiving attention as a promising technology. The principles of such a CMUT will be described. When an ultrasonic wave is transmitted, a voltage obtained by superimposing a minute alternating-current (AC) voltage on a direct-current (DC) voltage is applied across the lower and upper electrodes. As a result, the vibrating membrane is vibrated and an ultrasonic wave is generated. When an ultrasonic wave is received, the vibrating membrane is deformed by the ultrasonic wave, so that the capacitance formed between the lower and upper electrodes changes because of the deformation of the vibrating membrane and a signal resulting from the change in capacitance is detected. General capacitive electromechanical transducer apparatuses include a plurality of elements, in each of which a plurality of cells that are electrically connected to each other are electrically connected in parallel with each other. With such a configuration, the reception sensitivities of the elements may vary. A method in which sensitivity correction of the variations is performed has been proposed (see PTL 1). In this method, a control unit electrically adjusts an output signal in such a manner that the difference between output signals (the difference in sensitivity) resulting from conversion performed by ultrasonic detection elements becomes smaller.
The reception sensitivity of each of the cells or elements is inversely proportional to, for example, the square of the space (gap) between the upper and lower electrodes. Thus, if gaps between the upper and lower electrodes for the cells or elements vary, the reception sensitivity of the CMUT varies from cell to cell or from element to element. As a method for forming a gap for a capacitive electromechanical transducer apparatus, a method is generally used in which a sacrificial layer is arranged so as to have almost the same thickness as a desired interelectrode gap, a vibrating membrane is formed on the sacrificial layer, and then the sacrificial layer is removed to form the gap.